The foliar change in two species of Melaleuca (Myrtaceae): a morpho-anatomic and ontogenetic approach

Abstract

In Melaleuca minutifolia mature leaves are linear, in Melaleuca micromera scale-like and peltate. This peltation is caused by swollen basal mesophyll without meristematic fusions. Thus, a novel peltation strategy is described. The foliar change of two Australian Melaleuca species (Myrtaceae) was investigated in different ontogenetic stages from cotyledons up to the mature leaf types. In both taxa (M. minutifolia and M. micromera), a strong leaf reduction occurs. While in M. minutifolia the mature leaf type is linear, inverse bifacial and strongly adpressed to the shoot axis, in M. micromera it is scale-like, aequifacial, minute and peltate. The mature leaf type of M. micromera is one of the rare examples of peltation among woody species. The ontogenetic studies on the mature peltate leaf type of M. micromera showed that a shield-like leaf base and typical peltate leaf morphology can be formed, not exclusively by meristematic fusions, as is the case for peltate leaves in the classical sense, but instead by strongly swollen mesophyll, particularly in the basal part of the leaf. While most species with peltate leaves are herbaceous perennials from very moist to aquatic habitats and lack reduced leaves, the four Melaleuca species known with the M. micromera type of peltation are from habitats showing either seasonal water stress, soil infertility or both and all show extreme leaf reduction. Given our morpho-anatomical data, it is clear that, as well as the classical type of peltation, a novel, second strategy for achieving peltation needs to be distinguished. The two types are correlated with quite different environmental conditions.